Peah 5:1: A stack of grain underneath which Leket [individual stalks that fall during harvest, which must be left for the poor to glean] was not collected, everything touching the ground is for the poor. If wind scatters the sheaves, he estimates how much Leket [that field would make and gives [that amount] to the poor. Rabban Shimon ben Gamliel
says: He gives to the poor the amount that [normally] falls [in such a field].

Peah 5:2 :An [unharvested] ear of grain [in an area already harvested] the tip of which can touch the standing crop - if it can be harvested with the standing crop, it belongs to the property owner; if not, it belongs to the poor. If an ear of grain that is Leket is intermixed in a stack, one ear must be tithed and given to him [the poor man]. Rabbi Eliezer
said: And how does this poor person exchange for something that never came into
his possession? Rather, he grants him the entire stack and tithes one ear and
gives it to him [and then repossesses the stack].